In the prior art, a zoom lens, which is built in single-lens reflex cameras used with 35 mm film for versatile purposes of altering a focal length from the wide-angle to the telephoto, is comprised of four groups of component lenses to attain a high variable power in practical use where the four groups of component lenses include the first lens group of positive refractivity closest to the subject, the second lens group of negative refractivity second closest to the subject, the third lens group of positive refractivity, and the fourth lens group of positive refractivity farthest from the subject.
Such prior art high variable power zoom lens of four-lens-group (+, −, +, and +in refractivity) configuration suitable for use with 35-mm film has been disclosed by way of example as being about 3 to 4 in F number at the wide-angle end, about 6 to 7 in F number in the telephoto end, approximately ×10 in variable power ratio, and reduced in both dimensions and weight (see Patent Document 1 listed below). This high variable power zoom lens has four groups of component lenses which include the first lens group of positive refractivity closest to the subject, the second lens group of negative refractivity second closest to the subject, the third lens group of positive refractivity, and the fourth lens group of positive refractivity farthest behind from the subject, and in varying the power range from the wide-angle end to the telephoto end, the first and second lens groups go apart to leave a greater air gap therebetween, the second and third lens groups come closer to have a reduced air gap therebetween, the third and fourth lens groups come closer to have a reduced air gap therebetween, and the first, third and fourth lens groups move toward the subject although only the second lens group moves in focusing.
Other types of the prior art four-lens-group high variable power zoom lens for 35-mm film are disclosed which are hyper-magnifying power as high as ×12 for the photographing purpose, having additional capabilities of inner-focusing to take a close-up shot and of anti-vibration without compromising its quality performances (see Patent Document 2). The zoom lens is of a type that a sequence of its component lens pieces assume either positive, negative, negative, positive, negative, positive attributes of refractivity or positive, negative, negative, positive, positive attributes of refractivity, and a group of lenses third closest to the subject, namely, the third lens group is dedicated to the focusing while the fifth lens group supports as an anti-vibration means.
Requirements for such great capabilities and quality performances are a power of this anti-vibration lens group in combination with a power of the focusing lens group when the hyper magnification is desired to attain.
Other types of the prior art four-lens-group high variable power zoom lens for 35-mm film are those which serve as an anti-vibration zoom lens of high performance and compact body suitable for photography and videography, having an optical arrangement of the first lens group G1 of positive refractivity closest to the subject, the second lens group G2 of negative refractivity second closest to the subject, the third lens group G3 of positive refractivity, the fourth lens group G4 of negative refractivity, and the fifth lens group G5 of positive refractivity farthest from the subject. In zooming by altering a position relative to the subject from the wide-angle end to the telephoto end in the anti-vibration zoom lens where all adjacent pairs of the lens groups G1, G2, G3, G4 and G5 vary a distance between them, the third lens group G3 consists of a plurality of component lens pieces one of which is a composite lens L3A having a negative lens piece L3AN and a positive lens piece L3AP bonded together, and the composite lens L3A solely moves in a direction almost perpendicular to the optical axis to serve as the anti-vibration lens, which is disclosed as an arrangement for compensating for a blur of a focused image (see Patent Document 3).
Also, in the prior art, disclosed is a fish-eye lens of 10 mm in focal length suitable for digital cameras (see Patent Documents 4 and 5).
Patent Document 1: Japanese Patent Preliminary Publication No. 2003-241097
Patent Document 2: Japanese Patent Preliminary Publication No. 2003-329933
Patent Document 3: Japanese Patent Preliminary Publication No. 2004-212611
Patent Document 4: Japanese Patent Preliminary Publication No. 2004-69768
Patent Document 5: Japanese Patent Preliminary Publication No. 2004-126522
The prior art zoom lens, when used with a single-lens reflex camera suitable for 35-mm film, serves for versatile purposes of altering a focal length from the wide-angle to the telephoto of the zooming range. As newly developed ones of electronic still cameras, or namely, digital still cameras, APS cameras of a smaller imaging plane than a 35-mm film frame have been developed, for example. As for the high variable power zoom lens as disclosed in Patent Document 1, reducing an angle of view while a focal length is unchanged, the resultant angle of view is excessively small for the zoom lens to function as a wide-angle zoom lens. When a curvature, a distance between adjacent lens groups, and other factors are reduced in proportion with one another to have a reduced imaging plane, however, the focal length is decreased at a proportional reduction rate to the imaging screen. As a consequence, a backfocal distance is reduced, and therefore, the zoom lens can no longer be attached to a camera body that has a flange back dimensioned equivalently to that of 35-mm film cameras.
Similarly in the high variable power zoom lenses as disclosed in Patent Documents 2 and 3, reducing the angle of view as small as the APS size while the focal length is retained as it is, the resultant angle of view is too small for the zoom lens to function as a wide-angle zoom lens.
The present invention is made to overcome the above mentioned disadvantages of the prior art high variable power zoom lens of the four-lens-group configuration, and accordingly, it is the subject of the present invention to provide a high variable power zoom lens that ensures a flange-back or a backfocal distance required for typical 35-mm film cameras while keeping an angle of view sufficient to cover the wide-angle for an APS-size imaging plane dimensioned smaller than 35 mm.